Stopping the Islamic State's money sources is key to degrading the extremists' ability to wage war, Treasury Secretary Jack Lew said Friday, one day after the United States and Russia joined forces at the United Nations to stop the flow of money to the terrorists' operations.

"They need a lot of money to support the fighters they put in the field, and if we can degrade their funding sources, we can degrade their efforts," Lew told MSNBC's "Morning Joe" program.

On Thursday, the United Nations, during a meeting of the Security Council, adopted a resolution to that will place sanctions on ISIS' sources of financing, in the same way funding was blocked for al-Qaida in the past, reports CNN.

"The key leverage points are getting the whole world together to make that statement," said Lew. "We have pretty effectively shut ISIS off from the international financial system. We now need to tighten the screws."

Lew said the United States has already been successful cutting ISIS off from formal financial sources, but now needs to cut off the "informal financial system."

"We can shut down some of their funding," he said. "It doesn't have to be all of their funding, [but it] forces them to make tough choices."

For one, coalition forces have been able to take out 400 oil tankers that were moving oil through ISIS-controlled territory, and strikes on oil infrastructure have been going on for some time.

"There are some parts of the oil infrastructure that are more formal than others," he said. "As others have pointed out to me, don't think pipeline, think hoses. The infrastructure is not all the big industrial infrastructure, but we are hitting them at the places where they refine oil, where they transport oil and we need to do more."

ISIS brought in an estimated $900 million last year, according to reports, and its funding sources are evolving substantially all the time, said Lew.

"At the beginning what they did, they conquered territory," said Lew. "They took the banks and stole the money. That was like a sinking fund they used. They then diversified and moved into oil and extorting money. That's why the effort on oil is so important. We've gotten more effective as we've gotten more ability to strike at the heart of it."

Stopping the Islamic State's money sources is key to degrading the extremists' ability to wage war, Treasury Secretary Jack Lew said Friday, one day after the United States and Russia joined forces at the United Nations to stop the flow of money to the terrorists' operations.